Kirby sentria shampooing not right

I have seen others use their kirby sentria shampoo system and it puts out loads of suds where as mine just puts out a little soapy water. I have the switch all the way on and the green arrows are lined up on the front. I am moving slowly while running the kirby but nothing seems to work. when i bought this thing they used it once for me and it had loads of suds but i've never been able to get it to work right. Any help would be appreciated. Thanks